Electrochemical cells in general involve reduction-oxidation reactions in separated half-cells that are appropriately connected for ionic flow as well as electrical flow across an external circuit. Batteries and fuel cell produce useful work in the form of the electrical flow across a load generated from the reduction-oxidation reactions. In other electrochemical cells, a load is applied to the cell to induce desired chemical reactions at the electrodes to form desired chemical products. Fuel cells differ from batteries in that both the reducing agent and the oxidizing agent can be replenished without dismantling the cell. Fuel cells and in some cases batteries can comprise individual cells stacked in series to increase the resulting voltage. Adjacent cells connected in series can have an electrically conductive plate, e.g., a bipolar plate or electrical interconnect, linking adjacent cells. Since the reactants of a fuel cell can be replenished, appropriate flow paths can be integrated into the cell.
Several types of fuel cells have gained recognition as distinct classes of fuel cells that are distinguishable from each other due to the nature of their construction and the materials used in their construction. Particular fuel cell designs introduce specific challenges in material performances. Common features generally found in different fuel cell designs involve the flow of fuel and oxidizing agent for long-term performance with appropriate design consideration for heat management, electrical connection and ionic flow. Different fuel cell designs differ from each other in the construction of the electrodes and/or electrolyte, which separates the electrodes, and in some cases the particular fuel. Many fuel cell designs operate with hydrogen gas, H2, although some fuel cells can operate with other fuels, such as methanol or methane.
Several types of hydrogen fuel-based fuel cell differ in the electrolyte within the fuel cell. For example, proton exchange membrane fuel cells have a separator that conducts effectively only protons, i.e., hydrogen cations, to maintain electrical neutrality. Phosphoric acid fuel cells use phosphoric acid as the electrolyte, which also conducts protons. Molten carbonate fuel cells use molten mixed carbonate salts as the electrolyte in which the carbonate ions are transported through the electrolyte to maintain electrical neutrality. Solid oxide fuel cells use a ceramic separator, such as yttrium-stabilized zirconia, which transport oxygen ions. The conventional operating temperatures generally are dependent on the electrolyte material, with proton exchange membrane fuel cells operating at temperatures on the order of 80° C., phosphoric acid fuel cells operating at temperatures on the order of 190° C., molten carbonate fuel cells operating at temperatures on the order of 650° C. and solid oxide fuel cells operating at temperatures on the order of 650° C. to 1000° C. The fuel suitable for a fuel cell generally depends on the catalyst material, electrolyte composition, operating temperature and other performance properties.
In addition to hydrogen based fuel cells, direct methanol fuel cells operate with methanol directly used as the fuel. These cells can operate alternatively with either liquid methanol or vapor methanol. These cells generally are formed with polymer separators and liquid ionic electrolytes. Catalyst particles are generally included in the anodes and cathodes, respectively.